The together-again couple had such a love-in at the Hard Rock Hotel during last weekend’s Lollapalooza festival that they didn’t attend a Belvedere Nights event, even though the sponsors paid for their first-class flights to Chicago and VIP festival tickets.

Finally the pair — who insist they didn’t know they were expected to appear — paid for their hotel room and incidentals after the furious event organizer phoned Szohr and told her he’d cancel their return flights.

A source told us, “Jessica was invited and took Ed as her plus-one. They were flown first-class and given two rooms, and her talent wrangler, Aaron, agreed they would appear at night or at one of the day lounges.

“But they never showed up,” our insider continued, “and instead they hit the minibar and ordered room service. The bill for food and booze for three nights was over $830. Publicists were calling their room and banging on their door.

“Eventually this blew up into a big argument on Sunday, when the event boss phoned Jessica and said he was canceling their flights. She said ‘We don’t need to do this. We can pay our own way.’ They left that night, and Jessica paid for everything.”

The pair, who did emerge to see bands including The Strokes and Edward Sharpe and the Magnetic Zeros, split up earlier this year over Szohr’s flirting with Marco Minuto, but recently started reconciling.

A rep for Szohr insisted she had no idea they were supposed to appear at the Belvedere event: “If Jessica had known, she would have gone. She was never told until Sunday, and as a result she agreed to pay for everything herself and left. There was no fight.”

Westwick’s rep added, “Ed went as Jessica’s plus-one and had no idea about any agreement or photographs he had to take. He always honors any agreements.”